This invention relates to substrates for large area electronic devices such as flat panel display devices, for example, liquid crystal display devices, and other devices comprising thin film circuit elements carried on substrates.
Liquid crystal display devices typically comprise two planar glass substrates which are sealed together around their peripheries with a gap between their facing surfaces in which liquid crystal material is contained. On the facing surfaces of the two substrates electrodes are provided to which electrical potentials can be applied in order to obtain required display effects. The substrates usually also carry alignment layers and, in the case of a colour display device, a colour filter layer. In simple display devices the electrodes may define display segments providing, for example, alpha-numeric display information. In other display devices, for example as used for datagraphic display screens for computers, the electrodes may define a row and column array of display elements which are connected to sets of row and column address lines. In an active matrix display device each display element can have an associated switching device, usually in the form of a TFT (thin film transistor) or a TFD (thin film diode) which devices are carried on the inner surface of one of the substrates and connected to address lines. Glass sheets used for the substrates are relatively inexpensive and generally compatible with the processing technology used to provide the electrodes and other required layers thereon. However, the weight of the glass sheets can be a problem, particularly in larger area display devices and those used in portable equipment such as laptop computers and PDA""s where the glass substrates can constitute a significant part of the overall weight of the equipment. It is known also to use plastic substrates, for example polyimide, PET or PES polymer materials, but then there may be problems in making, for example, stable liquid crystal cells with such plastic substrates because they exhibit flexibility and can cause difficulties during the lithographical processing of films deposited thereon, as required for electrodes, address lines and switch element layers for example, due to their poor dimensional stability. Other known kinds of large area electronic devices similarly using glass sheets as substrates for carrying thin film circuit elements include sensing arrays, for example 2D image sensing arrays comprising a matrix of diode light sensing elements such as described in U.S. Pat. No. 5,349,174 fingerprint or touch sensing arrays as described in U.S. Pat. Nos. 5,325,442 and 5,270,711 and thin film memory array devices as described in U.S. Pat. No. 5,272,370.
It is an object of the present invention to provide a substrate for use in large area electronic devices which overcomes the aforementioned problems at least to an extent.
According to one aspect of the present invention, there is provided a substrate for carrying on an insulating surface thereof thin film circuit elements in a large area electronic device which is characterised in that the substrate comprises a thin glass sheet bonded to a layer of rigid, cellular material.
According to another aspect of the present invention, a large area electronic device, such as a matrix display device or sensing array device, having a substrate on which thin film circuit elements are carried is characterised in that the substrate comprises a thin glass sheet on one side of which the thin film circuit elements are carried and whose other side is bonded to a layer of rigid, cellular, material.
By replacing a conventional sheet of glass with such a composite structure a considerable weight saving can be achieved. The glass sheet used in the composite structure can be significantly thinner since the structural strength and rigidity required for the substrate is provided by the cellular material which itself, being cellular, can be of light weight compared with, volume for volume, glass material. Typically, the glass sheet may be around 0.1 mm thick compared with around 0.7 to 1 mm thickness for a conventionally used glass substrate. As one side of the laminate structure is constituted by solid glass, a high quality, electricallyxe2x80x94insulating, surface suitable for use as a base on which to form thin film circuit layers is still available.
The rigid, cellular material preferably comprises an aerogel. A substrate formed of aerogel material alone would be rough, as well as porous, and therefore not suitable for use as a substrate for large area electronic device applications, but has the rigidity and strength required, while being of relatively low weight, so that when combined with a thin glass sheet, and comparatively much thicker, the resulting combination satisfies the requirements for such a substrate. The composite substrate can readily be made by forming a layer of aerogel material directly on one side of the thin glass sheet. Aerogel materials are silicate sponges produced by a sol gel process which have been developed principally for their thermal insulation properties for use for example in windows and can be made with solid volume fractions as low as around 1% or even less which means they have low weight as well as good thermal insulation properties. Because of the common nature of their constituents the physical properties, such as the thermal expansion coefficients, of the component layers of the substrate are similar. Importantly for display device and other applications where substrate transparency is required, the transparency of the aerogel layer is reasonably good as the cells of their structure are small, typically having a dimension of a few nanometres to tens of nanometres. Such a cell structure would not result in any strong scattering of light.
For increased robustness the aerogel layer preferably has a higher than usual density. In a preferred embodiment, the thickness of the glass sheet is around 0.1 mm. Sheets of electronic grade glass having high quality surfaces suitable for use as substrate surfaces in large area electronic devices and of such thickness are available commercially. The thickness and density of the aerogel layer are then selected to provide the required rigidity and strength to the substrate. To this end, the aerogel layer desirably has, for example, a solid content of around 10-30% by volume and a thickness of around 0.5 to 1 mm. Besides being of comparatively low weight, around 10-30% of a similarly sized solid glass sheet, such an aerogel layer also offers high dimensional and mechanical stability.
The cellular layer need not be bonded directly to the glass sheet but may be bonded through an intermediate layer. In a particular embodiment, the substrate may include a microlens array between the glass sheet and the cellular layer. Microlens arrays, which term is intended herein to include microprism arrays, are used in certain kinds of matrix display devices, such as autostereoscopic display devices, to control or direct light input to or output from the display pixels. Such microlens arrays are often carried on a separate glass plate which is attached to the display device. The incorporation of a microlens array in the substrate to be used for the display device therefore leads to a further beneficial weight saving by eliminating the requirement for a further, separate, substrate. Moreover, because the microlens array is provided in the substrate which is to carry thin film elements of the pixel array, accurate alignment between the microlens elements and the pixels can be ensured. Further, optical advantages can be obtained due to the microlens array being positioned adjacent to the thin glass sheet and therefore much closer to the actual pixels. Such a substrate can also be used other large area electronic devices such as for example image sensing devices in which microlens elements are used to direct or concentrate incoming light onto photosensitive elements.
The microlens array can conveniently be formed using known materials and techniques directly on the surface of the glass sheet, but preferably by a resin molding process, and then covered by the deposited cellular layer. Such a microlens array may have an overall thickness of around 10 xcexcm and so the thickness of the substrate is not unduly increased. Optical performance of the microlens array need not be affected significantly through being embedded. Depending on the refractive indices of the materials used to form the microlens array and the cellular layer only a small loss in optical power is likely.
The form of the microlens elements may be varied and of any known shape, for example spherical or elliptical, toroidal or cylindrically elongate, depending on their intended purpose. The array may be provided in the form of a continuous layer in which adjacent microlens elements join one another or the elements may be spaced slightly apart from one another. In the former case, the microlens array physically separates the cellular layer from the glass sheet and bonding of the layer to the sheet is achieved via the array. In the latter case, the cellular layer can extend around the individual microlens elements and contact the surface of the glass sheet in the spaces between the elements so that bonding between the cellular layer and the glass sheet is achieved directly in part.
It is envisaged that cellular materials other than an aerogel material could be utilised. In the case where transparency is desired, a cellular glass structure may be used. Certain glass compositions undergo phase separation on solidification resulting in an interpenetrating, open cell, network. This network can be opened up by the selective etching and removal of one phase. The resulting structure is porous, and hence light weight. This glass cellular structure could be formed as a sheet and bonded to the thin, solid, glass sheet to provide the composite substrate, the solid sheet being, for example, of a higher grade glass material more suited for use with thin film processing technology. However, it may be possible to use a multi-phase composition sheet and etch it selectively through part only of its thickness so as to leave a relatively thin, solid layer adjacent one side. Although the nature of the solid layer, and hence its compatibility with subsequent processing stages, would then be dictated by the original glass composition, it may be adequate for certain kinds of applications.